1. Field of the Invention
The present invention relates to a mobile communication system and, in particular, to handover control in a mobile communication system.
2. Description of the Related Art
One of communication schemes used in mobile communication systems is CDMA (Code Division Multiple Access). In a mobile communication system that uses CDMA, multiple signals in the same frequencies can be distinguished from each other by using multiple spread codes. Therefore, the same frequency can be used to form multiple cells.
In such a CDMA mobile communication system, a mobile station receives signals having a certain frequency and extracts a desired signal from among the received signals by performing despreading using one spread code. The mobile station can receive signals from multiple cells at a time by using multiple spread codes. The mobile station switches between cells from which the mobile station is to receive a signal while receiving signals from multiple cells at one time, thus implementing seamless handover.
In order to allow mobile stations to determine handover, a radio base station sends different CPICH (common pilot channel) signals for different cells. A mobile station receives a CPICH signal, identifies the cell that sent the CPICH signal, and compares the power of the received CPICH signal with a threshold. The comparison is made in order to determine the line conditions of the path of the CPICH signal. Based on the result of the comparison of each received CPICH signal with the threshold, the mobile station determines a cell from which the mobile station should receive a signal. The cell from which a mobile station receives a signal will be referred to as handover target cell. Once the mobile station determines a cell from which the mobile station will receive a signal, namely a handover target cell, the mobile station adds and/or deletes a cell on the basis of the determination. Any value of the threshold within the limits of signal power at which signals can be received can be set.
However, the transmission power of CPICH signals from radio base stations are not equal among all cells. When the transmission power of CPICH signals differs among cells, a mobile station cannot properly determine the line conditions of paths on the basis of the power of CPICH signals that the mobile station received.
On the other hand, an uplink dedicated channel signal directed from a mobile station to cells, for example, is transmitted at a common level, and the transmission power does not differ in the cells. The received power of the uplink dedicated channel signal varies depending on losses on paths. Because a mobile station makes determination regarding handover on the basis of the received power of CPICH signals, it is possible that a path with a large loss is selected as a handover target. If this occurs, the power of the signal of the uplink dedicated channel received at the radio base station is weakened.
Like a mobile station, a radio base station can combine signals received in multiple cells. When a path that has a small loss is included in a set of multiple selected paths, the radio base station can receive a dedicated channel signal having relatively good quality. However, when all the selected paths have large losses, possibly the radio base station cannot receive a dedicated channel signal that has a good quality.
FIG. 1 is a diagram illustrating the relationship between the power of a received CPICH signal and path loss. It is assumed here that CPICH signals from first and second cells reached a mobile station. PT1 in FIG. 1 represents the transmission power of the CPICH signal of the first cell and PT2 represents the transmission power of the CPICH signal of the second cell. PR1 represents the power of the CPICH signal from the first cell received at the mobile station and PR2 represents the power of the CPICH signal from the second cell received at the mobile station. L1 represents path loss between the first cell and the mobile station and L2 represents path loss between the second cell and the mobile station.
When the mobile station is at point A, PR1>PR2. Accordingly, the power of the signal received at the mobile station through the path from the first cell is greater than that of the signal through the path from the second cell and therefore the power of the signal through the path from the first cell seemed to be in a better condition. However, actual path losses are expressed as L1=PT1−PR1 and L2=PT2−PR2. As can be seen from FIG. 1, L1>L2, that is, path loss between the mobile station and the first cell is greater than path loss between the mobile station and the second cell. In spite of the fact that the loss in the uplink dedicated channel signal over the path between the mobile station and the second cell is greater than the loss in the uplink dedicated channel signal over the path between the mobile station and the first cell, the power of the signal received through the path between the mobile station and the second cell is higher.
On the other hand, unlike a common channel CPICH signal whose transmission power is fixed, the downlink dedicated channel signals from the cells to the mobile station are amplified to different degrees by transmission power control. When a path on which a large loss occurs is selected as a handover target, transmission power at a radio base station is controlled to a high level or the power of a signal of a downlink dedicated channel received at a mobile station can be decreased.
There is a CDMA mobile communication system that, unlike the system described above, allows a mobile station to use a downlink dedicated channel signal, in addition to a common channel signal, to determine a handover target cell (see Japanese Patent Laid-Open No. 2006-54625).
However, this technique has the following problem.
In the mobile communication system described in Japanese Patent Laid-Open No. 2006-54625, degradation of communication quality caused by the difference between a common channel signal and a downlink dedicated channel signal can be reduced because handover is made to a cell that transmits a downlink dedicated channel signal of a better quality. However, degradation of communication quality caused by the difference in transmission power of a CPICH signal, which is a common channel signal, between cells described above sometimes was not reduced.
An object of the present invention is to provide a mobile communication system capable of reducing degradation of communication quality caused by the difference between cells in transmission power of a common channel signal.